Combustion devices, including gas turbine engines, must comply with ever increasing regulatory emissions standards while maintaining or optimizing performance and cost. Mandated emissions regulations to reduce oxides of nitrogen (NOx) are the main driving force behind the development of new and costly technologies involving premixed combustion, catalytic systems, and post combustion clean-up. To comply with these emission standards, fuel-lean and fuel-rich catalytic systems have been proposed.
Lean-burning catalytic systems have advanced significantly to provide a stable (dynamic-free) burn with low NOx emissions. However, such systems require preheating of the gas flow as the catalyst will not light at standard combustor inlet temperatures. To preheat the gas to necessary inlet temperatures, a pre-burner is located upstream of the catalyst. Additionally, if the temperature rise required by the engine cycle is higher than what the catalyst can provide, then a post catalytic burner is also required. These preburners and post burners can be the primary source of NOx emissions for lean-burn catalytic systems.
Fuel-rich catalytic systems are much more active at lower temperatures than the fuel lean catalytic systems. However, even the fuel-rich catalytic systems cannot operate below a certain engine rpm or temperature, and accordingly, such fuel-rich systems may also face a light-off problem. Should the catalytic system fail, such fuel-rich systems begin to resemble, in terms of performance, a conventional lean premix system and become incapable of sustaining combustion for the entire range of engine operation. Additionally, conventional fuel-rich catalytic systems do not provide mechanisms or methods for controlling flame aerodynamic stability, for controlling the combustion process, for providing ignition sources, or for introducing diffusion fuel streams for combustion at engine operating conditions beyond the range of catalytic, device operability, or, in the case when the catalytic device fails and the combustion regime reverts to conventional lean premixed combustion, for the utilization of diffusion fuel in abating dynamic fluctuations associated with lean premixed combustion.
Therefore, there is a need for a system and method that can operate in a fuel-rich catalytic system mode or a diffusion fueled mode while controlling flame aerodynamic stability and the combustion process, and abating the dynamic fluctuations associated with lean premixed combustion, should the catalytic system fail.